Bentonite is usually obtained initially as an alkali earth compound, for instance a calcium and/or magnesium form of bentonite. In order that it can perform satisfactorily, for instance in paper making, it is necessary to activate the bentonite by subjecting it to ion exchange whereby calcium and/or magnesium is replaced by sodium or other alkali metal or ammonium ion.
The normal way of doing this is to mix the alkaline earth bentonite clay with an aqueous activating solution containing an alkali metal or ammonium compound. For instance the clay may be provided as a powdered pre-blend of the alkaline earth metal clay and the alkali metal or ammonium compound and this pre-blend may be mixed into water and allowed to stand for sufficient time for ion exchange to occur whereafter it is diluted for use. Alternatively the quarried alkaline earth bentonite can be added direct to an aqueous activating solution with mixing.
In practice, the activating compound which is conventionally used is sodium carbonate or sodium bicarbonate, typically in an amount of around 5 to 10% by weight based on the weight of clay. The activation is conducted at the natural pH of the dispersion, around pH 10.
Other electrolytes are described in JP-A-6445754, as being added to promote dispersion. It is also known in U.S. Pat. Nos. 4,613,542 and 4,624,982 to include sodium polyacrylate. Concentrates of bentonite with various alkali metal compounds are also described in EP 485124 and U.S. Pat. Nos. 5,266,538 and 5,391,228. The bentonite that is introduced into these concentrated slurries can initially be in the form of sodium bentonite but EP 485,124 and U.S. Pat. No. 5,391,228 also mention making slurries by blending calcium bentonite and sodium carbonate activator with water and sodium chloride. Paper making processes using bentonite with an anionic organic polymer are also described in EP 373306 and U.S. Pat. No. 5,015,334.
One problem with known activated bentonites (for instance the activated bentonite powders which are commercially available) is that the sodium carbonate or other activator which is included in them causes the bentonite (when mixed with water) to give a relatively high pH, for instance above pH 9 and often around pH 10. Accordingly it is necessary to handle the bentonite with the care associated with materials which do generate a pH of around pH 10.
Another problem is that the conventional sodium carbonate activator undergoes ion exchange during the activation process to form calcium carbonate, with the result that inorganic scale may be formed and deposited from aqueous compositions of these activated bentonites. This is a particular problem when the water in which the bentonite is being dispersed is, naturally, relatively hard.
Another difficulty with known activated bentonite powders is that it is sometime found to be rather difficult to make a stable aqueous dispersion of the powders. Again, it has been found that these difficulties of achieving a stable dispersion increase with increasing hardness of the water. Thus, when using relatively hard water, for instance having a hardness of above 10.degree. dH and generally in the range 15.degree. to 50.degree. dH, various problems can arise.
Further, the performance of the bentonite, especially when used in paper making, can be less effective at lower pH values than at higher pH values, and in particular bentonite activated in conventional manner tends to be less effective in acid paper making processes (for instance when the cellulosic suspension has a pH of below 6.5 and often in the range 4.2 to 5.5) than when the suspension has higher pH values.
We have now discovered a new range of activators which can give improved performance.